Few people, if any, look forward to colonoscopies. They're annoying to prepare for. (Liquid diet? No thank you.) They're invasive. (It's a camera. In your butt.) They're scary. (Even if it's just a routine screening, there's always a chance your doctor will find something up there that requires medical attention.) They may even be over-prescribed. And, as we learned at last week's Ig Nobel Awards Ceremony, sometimes colonoscopy patients go boom. Because we know you're curious, here's how that happens.

"Colonic gas explosions," as their name implies, occur inside a patient's colon. This stretch of guts is commonly known as the large intestine and makes up the last five feet or so of your intestinal canal, beginning at the bottom end of your small intestine and ending at your anus. Unlike your small intestine, which is largely responsible for digesting food and absorbing nutrients, one of the large intestine's most important roles is storing and eliminating fecal matter. It's also where farts are born. (And if the Internet has taught us anything, it's that farts can be flammable. Also, this.)

Housed in your large intestine are hundreds of species of beneficial bacteria that help digest those bits of food your small intestine misses. In the process, these bacteria generate a variety of gases, including carbon dioxide, oxygen, nitrogen, hydrogen and methane, to name a few. Some of these gasses are odorless. Others, (particularly ones containing sulfur), are not. Two of them — hydrogen and methane — also happen to be combustable. Given that the average person's bacteria produce between 1 and 4 pints of gas per day, a five-foot stretch of colon can pack some formidable explosive potential.

Fortunately, it takes a pretty special set of conditions to detonate an intestinal bomb, so only rarely does this potential actually get unleashed. According to a team of researchers led by Emmanuel Ben-Soussan (who last week was awarded an Ig Nobel prize in medicine for advising doctors on how to avoid gut-combustion), an explosion of colonic gasses requires three things:

Your bacteria provide the first two; electrocautery — a technique that uses heat to remove potentially cancerous intestinal growths known as polyps — provides the third. The perfect colonic storm would comprise a high concentration of hydrogen and/or methane (greater than 4% or 5%, respectively), plenty of oxygen and a piping-hot electrocautery tool.

Concentrations of hydrogen and methane in the colon can vary considerably (0.06%—47% and 0%—26%, respectively, according to this study). Taking these thresholds into consideration, it is estimated that almost half of colonoscopy patients with unprepared large intestines harbor potentially explosive concentrations of hydrogen and methane in their bowels.

And yet, a survey of the medical literature conducted by Ben-Soussan's team turned up just 20 cases of colonic gas explosion between 1952 and October 2006, only one of which was fatal. Why such a small number? Because if you read the last paragraph closely, you'll notice that the cited hydrogen and methane concentrations are high in unprepared (i.e. uncleaned) large intestines; and adequate bowel cleansing, as you might expect, is pretty common practice in colonoscopies. (Still, not all colon-cleansing solutions are created equal. Mannitol, once widely employed in the preparation of the large intestine for colonoscopies, has been shown to increase hydrogen and methane excretion. Its use, unsurprisingly, has been largely discontinued.)

As Ben-Boussan and colleagues point out: "following bowel preparation with a combination of clear liquids, cathartics, and enema, mean concentration of hydrogen... and methane" generally fall well below their minimal explosive concentrations:

Quote:

An accumulation of colonic gas to potentially explosive concentrations due to poor colon preparation is considered an initiating factor in the complication of colonic gas explosion. Therefore, quality of bowel preparation as well as type of preparation and dietary restrictions are all essential for an uneventful therapeutic colonoscopy.

The upshot? While colonic explosion may be, as the researchers call it, "one of the most frightening [medical complications] of colonoscopy with electrocautery," it's also pretty damn rare. Here's to uneventful colonoscopies.